bytereader: fix peek value when scanning for 00 00 01 with non-0 offset

bytereader: fix peek value when scanning for 00 00 01 with non-0 offset

We would add the offset a second time in _scan_for_start_code()
when we found a result, but it's already been added to the data
pointer at the beginning of _masked_scan_uint32_peek(), so the
peeked value would be wrong if the initial offset was >0, and
we would potentially read memory out-of-bounds.

Add unit test for all of this.

https://bugzilla.gnome.org/show_bug.cgi?id=778365